A power converter is a power processing circuit that converts an input voltage waveform into a specified output voltage waveform. In many applications requiring a DC output, switched-mode DC/DC converters are frequently employed to advantage. DC/DC converters generally include an inverter circuit, an input/output isolation transformer and a rectifier on a secondary side of the isolation transformer. The rectifier within the converter generates a DC voltage at the output of the converter. Conventionally, the rectifier comprises a plurality of rectifying diodes that conduct the load current only when forward-biased in response to the input waveform to the rectifier. However, diodes produce a voltage drop thereacross when forward-biased. Given an escalating requirement for a more compact converter that delivers a lower output voltage (i.e. 3.3 V for a central processing unit, or "CPU," of a computer), it is highly desirable to avoid the voltage drop inherent in the rectifying diodes and thereby increase the efficiency of the converter.
A more efficient rectifier can be attained in converters by replacing the rectifying diodes with active switches, such as field effect transistors ("FETs"). The switches are periodically toggled between conduction and nonconduction modes in synchronization with the periodic waveform to be rectified. A rectifier employing active switches is conventionally referred to as a synchronous rectifier.
There are two classes of synchronous rectifiers. The first class of synchronous rectifier is conventionally referred to as "self-driven" synchronous rectifiers. Self-driven synchronous rectifiers presently enjoy widespread acceptance in power converters. In self-driven synchronous rectifiers, the biasing drive signals that control the synchronous rectifier switches are directly produced from the naturally-present voltages in the output circuit of the converter. The second class of synchronous rectifier is conventionally referred to as a "control-driven" synchronous rectifier. Contrary to self-driven synchronous rectifiers, the biasing drive signals that control the synchronous rectifier switches are produced by a regulation control circuit that determines the biasing of the main power switch or switches that constitute the inverter portion of the converter. Currently, control-driven synchronous rectifiers are not as widely used as self-driven synchronous rectifiers because of the additional regulation control circuitry required to drive the synchronous rectifiers. Also, maintaining the proper timing of the rectifier drive signals relative to the inverter drive signals can be difficult, thereby hindering the use of control-driven synchronous rectifiers.
However, control-driven synchronous rectifiers enjoy some distinct advantages over self-driven synchronous rectifiers. First, since the drive signals of the self-driven synchronous rectifier are produced by the naturally-present voltages in the output circuit of the converter, the amplitude of the drive signals to the synchronous rectifier are frequently of insufficient magnitude, thereby resulting in poor rectification of the resulting output voltage signal.
Second, since the drive signals of the self-driven synchronous rectifier are generated by the switching action of the inverter, there is limited latitude to advance the timing of the drive signals for the synchronous rectifier relative to the drive signals of the inverter. This limitation is especially disadvantageous when the operating conditions of the power converter vary over wide ranges. For example, during "partial" load or no-load operating conditions, the losses in some power-converter designs are excessive because the driven signals for the self-driven synchronous rectifier cannot be independently timed to drive the synchronous-rectifier switches at their most efficient point.
Therefore, control-driven synchronous rectifiers provide both controllable-amplitude drive signals and, with the use of delay circuits, completely flexible drive timing for the synchronous rectifier switches. While conventional control-driven synchronous rectifiers provide a mechanism to set a relative timing different of the drive signals with respect to one another, there is an additional concern that must be addressed.
In such control-driven synchronous rectifiers, the relative timing of the drive signals to the synchronous rectifier and the main power switches is fixed to maximize efficiency while keeping the stresses on individual components within acceptable limits. In some ways, however, the optimum drive timing for one set of operating conditions is different from the optimum drive timing for another set of operating conditions. For instance, a synchronous rectifier drive timing that produces maximum efficiency at a first load condition may produce excessive voltage stress on the rectifier switch at a second, lesser load condition. Conversely, when the timing is changed to lower the voltage stress at the second load condition, a loss of efficiency is liable to occur at the first load condition.
Accordingly, what is needed in the art is a drive circuit for a converter employing an inverter and a synchronous rectifier that adapts the delay between the drive waveforms supplied to the inverter and synchronous rectifier as a function of an operating condition of the converter to allow the converter to operate efficiently over a far wider range of operating conditions.